Method of dissolving metal oxides with di- or polyphosphonic acid and a redundant

ABSTRACT

A method of dissolving metal oxides using a mixture of a di- or polyphosphonic acid and a reductant wherein each is present in a sufficient amount to provide a synergistic effect with respect to the dissolution of metal oxides and optionally containing corrosion inhibitors and pH adjusting agents.

This invention was developed under contract from the DOEW-31-109-ENG-38.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of applicaton Ser. No. 08/111,782,filed Aug. 25, 1993 now abandoned, which is a continuation ofapplication Ser. No. 07/766,841, filed on Sep. 27, 1991 (abandoned),which is a divisional of application Ser. No. 07/516,436, filed Apr. 30,1990 now U.S. Pat. No. 5,078,894, issued Jan. 7, 1992.

This application is related to PCT International Patent Application No.PCT/US89/04857 which deals with complexing agents for solubilizingcertain heavy metals.

BACKGROUND OF THE INVENTION

The present invention relates to a synergistic composition fordissolving metal oxides and in particular iron oxides.

The dissolution of iron oxides is a very important technological processwith applications in a number of different fields. Effective dissolutionagents can be useful for (1) leaching of oxide ores in hydrometallurgyto recover metal values from the oxide ores; (2) removal of iron oxidecontaminants from non-metallic materials such as minerals (e.g. kaolinand silica), of importance for the glass and ceramic manufacturingindustry; (3) stripping of metal ions from extraction solvents or fromion exchange resins; (4) cleaning of metals prior to subsequentprocessing (e.g. removal of scale from a metal prior to rolling,anodizing, galvanizing or painting; (5) removal of deposits fromequipment in contact with hot water or steam (e.g. thermal powerequipment, heat exchangers and piping, and boilers); (6) cleaning ortreatment of rusty bolts, nuts and connectors; (7) removal of stains(iron oxide related) from fabrics; and (8) treatment of radioactivematerials including removal of a surface layer of oxides from steelequipment used for radioactive material processing or in nuclear plants,in order to facilitate the removal of radioactive contaminants (e.g. theactinide and lanthanide elements) present in or below the surface oxidelayers by subsequent application of an appropriate complexing agent;leaching of radioactive sludge; and leaching of radioactive soil.

It is known in the art that acids accelerate the dissolution of metaloxides; however severe conditions of concentration or temperature areusually required to achieve fast rates. These conditions present safetyhazards in handling and the potential for corrosion, if metals arepresent.

It is also known in the art that reductants can accelerate thedissolution of metal oxides, but the dissolution rates under moderateconditions are still low.

In general this can be seen in the following:

GB-1229582 ((Albright & Wilson Mfg. Ltd.-1971)- U.S. Pat. No. 3634257(1972) equivalent) describes the use of phosphonic acids to removeencrustations containing calcium carbonate or iron compounds from rigidsurfaces, e.g. stills and boiler tubes. The preferred acid is1-hydroxyethylidene-1,1-diphosphonic acid, which can also be used withother scale removing acids and with ammonia.

Albright & Wilson sells a product based on GB 1229582, BRIQUEST DP 13,for use as a railway carriage cleaner. It is described as a blend ofBRIQUEST ADPA (1-hydroxyethylidene-1,1-diphosphonic acid) andorthophosphoric acid specifically formulated for use withcarriage-washing machines.

Valverde N. and Wagner, C., Ber. Bunsenges Physik. Chem. 80(4) (1976)330 and Valverde N., Ber. Bunsenges. Physik. Chem. 80(4) (1976) 333describe experiments which show that the dissolution of iron oxide isaccelerated by decreasing the redox potential and attribute this toreduction of Fe⁺³ in the oxide lattice to give the more soluble Fe⁺².

Lu, Z. Y. and Muir, D. M., Hydrometallurgy 21 (1988) 9 describesexperiments in which an enhanced leaching rate of metal ferrites andiron oxides is found when HCl leaching solutions also contain Cu⁺¹ orSn⁺² as reductants.

Zinder, B., Furrer, G. and Stomm, W., Geochimica et Cosmochimica Acta 50(1986) 1861 describes acceleration of the rate of alpha - FeOOHdissolution by NaNO₃ in the presence of ascorbic acid.

Surana, V. S. and Warren, H. J., Trans. Inst. Min. Metal. Sec. C, 28(1969) C133-C139 and Kunda, W., Rudyk, B. and Mackiw, V. N., Bull. Can.Inst. Min. Metall. 61 (1968) 819 describe the use of SO₂ in dissolutionof iron oxides.

Warren, I. H., "Removal of Iron Oxide from Silicate Minerals" in M. E.Wadsworth and F. T. Davis (Eds.), Unit Processes in Hydrometallurgy,Vol. 1, pages 300-307, A.I.M.E., New York, 1964 describes the use ofsodium dithionite in dissolution of iron oxides.

Gorichev, I. G. and Kipriyanov, N. A., "Regular Kinetic Features of thedissolution of Metal Oxides in Acidic Media", Russian Chemical Reviews53(11) (1984) 1039 is a review article which discusses the effect ofredox couples on the rate of dissolution of metal oxides.

Azuma, K. and Kametani, H., Trans. Metall., Soc. AIME 230 (1964) 853indicates that the rate of dissolution of iron oxides increases with thestability constant of Fe⁺³ with the anion of the electrolyte.

Banwart, S., Davies, S. and Strumm, W., Colloids and Surfaces 39 (1989)303, describes the use of oxalic acid and L-ascorbic acid to dissolveiron oxide and shows an advantage for using the two together.

Gorichev, I. V., Gorsheneva, V. F., Kipriyanov, N. A. and Klyuchnikov,N. G., Kinetika i Kataliz 21 (6) (1980) 1422, describes the dissolutionof magnetite (Fe₃ O₄) with 1-hydroxyethylidene-1,1-diphosphonic acid andrefers to the potential use in removal of magnetite deposits and scalefrom steel.

U.S. Pat. No. 3,854,996 to Frost teaches the use of polyphosphonic acidsto remove magnetite (iron oxide) scale. To help with the action, the useof an oxidizing agent is recommended.

BRIEF DESCRIPTION OF THE INVENTION

Metal oxides and metal salts are dissolved by treatment with aqueoussolutions containing mixtures of di- or polyphosphonic acids andreducing agents, preferably sulfur based. Optionally, the solution cancontain corrosion inhibitors and pH adjusting agents.

The mixtures according to the present invention produce dissolutionrates which are substantially higher than those observed with either theacids or the reductants by themselves.

Of the di- and polyphosphonic acids,1-hydroxyethylidene-1,1-diphosphonic acid (HEDPA) is especiallypreferred for its high activity.

Preferred reductants are the sulfur-based reducing agents such as sodiumsulfite (Na₂ SO₃) and especially sodium formaldehyde sulfoxylate (SFS)and sodium dithionite (Na₂ S₂ O₄).

When used on some metal surfaces, it is expected that corrosion can be aproblem. Corrosion inhibitors or pH adjustors (to neutralize thenormally acid mixture or render it basic) can reduce corrosion problems.

The inventive mixtures can be used especially to remove radioactivecontaminants from equipment and tools used in handling and processingradioactive materials and in dissolving radioactive sludge and removingradioactive oxides and salts from contaminated soil where alternativemethods are not readily available.

DETAILED DESCRIPTION

The invention is based on the discovery that aqueous mixtures of di- orpolyphosphonic acids and reducing agents produce unexpectedly highdissolution rates as compared with the rates obtained for acids or forreducing agents alone, or mixtures thereof in general.

Reducing agents include sulfur-based reducing agents such as sulfurousacid, disulfurous acid, dithionous acid, hydroxymethylsulfinic acid(acid of SFS), hydroxymethyl sulfonic acid, and their salts; phosphorousbased reductants such as phosphorous acid, hypophosphorous acid andtheir salts; aldehyde or hydroxymethyl adducts thereof, such asformaldehyde, acetaldehyde, hydroxymethylsulfinic acid, hydroxymethylsulfonic acid, or their salts, tetrakis(N,N,N',N,-hydroxymethyl)urea,and a tetrakis(hydroxymethylphosphonium) salt. Nascent hydrogen(produced in situ) and water soluble salts forming Cr⁺² V⁺² and Cu⁺¹ arefurther examples of reductants for use in the present invention.

The synergistic effect is very pronounced in the case of di- orpolyphosphonic acids mixed with sulfur-based reducing agents, andespecially a combination of HEDPA and SFS. Very high rates were alsoobserved for hydrogen produced in situ (by reaction of Zn in an acidmedia). Both HEDPA and SFS by themselves produce very low dissolutionrates, but when combined, very fast dissolution rates are obtained evenat 25 degrees Centigrade. This permits safer, faster and easierprocessing while minimizing the risk of corrosion. Corrosion can befurther inhibited using pH adjustors or usual corrosion inhibitors.

For an acidic mixture as described above, a pH adjustor will normally bea basic substance. Thus, when pH adjustors are used, the acids will beat least partially converted to their salts. pH adjustors are, forexample, alkali metal or alkaline earth metal hydroxides such as sodium,calcium or potassium hydroxide. As the mixture is neutralized, most orall of the phosphonic acid will convert into the corresponding alkalimetal or alkaline earth metal salt. This conversion will besubstantially complete when the inventive mixture is adjusted into thealkaline pH range.

Other pH adjusting agents that can be used to advantage include ammoniumand various derivatives thereof e.g. alkyl, alkylene or alkanolderivatives. Examples of alkyl derivatives include a C₁ -C₃ alkyl amineand especially methyl amine, di- or tri-methyl amine. Alkylenederivatives include ethylene diamine and others corresponding to thealkyl compounds. Corresponding alkanol derivatives can also be used,including diethanoiamine for example.

One important use for this technology is in decontaminating tools,reactors and other equipment, which have been contaminated withradioactive materials (e.g. oxides and salts of actinide and lanthanideelements) in nuclear weapons and nuclear power operations. Although theinvention can be applied quite widely for removal or dissolution ofmetal oxides as noted above; for decontamination of radioactive metalsurfaces there are few practical alternatives available. Control ofcorrosiveness as by pH adjusting agents or corrosion inhibitors may beparticularly important in these uses, to avoid degrading difficult toreplace parts and apparatus.

In order to decontaminate a metal surface according to the invention,the surface oxide coating containing the radioactive contaminants isremoved. The best way to do this is to attack and dissolve the surfaceoxide coating directly. The surface oxide will in most cases be an ironoxide since most equipment is constructed from iron or iron-containingalloys. The radioactive contaminants may then be carried or washed intosolution with the iron oxide, or complexed with reactants to take theminto solutions.

A further advantageous use for this technology is for the retrieval andtreatment of high level nuclear waste, at present stored in single ordouble shell tanks at a number of nuclear sites. Many of these nuclearwaste solutions have been neutralized in the tanks by addition ofalkali, to reduce their corrosive power. This treatment has generatedlarge amounts of radioactive sludge in the tanks. In order to retrieveand further process these wastes to concentrate the high levelradioactive components and carry out permanent disposal, it will benecessary to leach and/or dissolve the sludge solids to release theradioactive metals into solutes.

A third use is in the leaching of radioactive or other metal (e.g.chromium) contaminants from contaminated soils. Leaching of soils isaccomplished by heap leaching or by processing in tanks.

Lead or manganese oxides can be dissolved by contacting with thecomposition of this invention.

Another use is for recovering of metal values from metal oxidecontaining materials by first dissolving metal oxides by forming acomplex with a mixture of a di- or polyphosphonic acid and a reductant,each being present in a sufficient amount to provide a synergisticeffect with respect to the dissolution of metal oxides; and thereafter,releasing the metal oxides from the complex, for recovering or disposal.Releasing the metal oxide is accomplished by destroying the complex suchas by treatment with an oxidizing agent. A useful oxidizing agent ishydrogen peroxide. The mixture for dissolving the metal oxide canfurther contain a pH adjusting agent or a corrosion inhibitor.

Other uses include treating of oxide ors (as by heap leaching or intanks) in hydrometallurgy to leach metal values from the oxide ores;removal of iron oxide contaminants from non-metallic materials such asminerals (e.g. kaolin and silica by batch processing in tanks); washingmetals with the inventive composition prior to processing (e.g. removalof scale from a metal prior to rolling, anodizing, galvanizing orpainting); (7) flushing of pipes, boilers, and etc. for the removal ofdeposits from surfaces in contact with hot water or steam by pumping themixture through the pipes; cleaning or treatment of rusty bolts, nutsand connectors; and (9) removal of stains (iron oxide related) fromfabrics by solubilizing the iron oxide and washing it away.

To illustrate the very effective oxide dissolving effectiveness of theacid/reductant mixtures of the invention, a synthetic iron oxidemineral, Goethite (alpha-FeOOH), is used as a model.

Experimental procedure

Radioactive synthetic Goethite samples, containing ⁵⁹ Fe at a level of1.7×10⁴ count per minute/mg of dried solid were prepared and used forthis and subsequent dissolution experiments.

The dissolution experiments were performed by contacting in screw-capglass tubes a weighed amount of radioactive Goethite (about 50 mg) withthe dissolving medium at a volume-weight ratio of 40 (2 ml for 50 mg).The size of the particles of iron oxide were estimated to be in therange of 60-120 mesh. The glass tubes containing the Goethite werethermally equilibrated in a heating bath and then the dissolvingsolution was added.

The mixture in the glass tubes was agitated using a magnetic stirrer, tosuspend the iron oxide in the dissolving medium. At various timeintervals tubes were withdrawn from the bath; rapidly quenched bycooling; and thereafter centrifuged to remove the particles of Goethitefor suspension.

The iron levels in the supernatant solutions were measured by measuringthe radioactivity of the supernatant solution with a gamma counter.First order rate constants and induction times for dissolution of theGoethite were calculated from the measurements by standard methods.

EXAMPLE 1

Samples of the Goethite were treated at 25 degrees Centigrade with 0.1Msolutions of each of the following reductants: hydroquinone, NH₂ OH HCl,SnCl₂, Na₂ SO₃, Zn(H₂), Na₂ S₂ O₄ and sodium formaldehyde sulfoxylate(SFS), or 1.0M samples of each of the following diphosphonic acids:1-hydroxyethylidene-1,1-diphosphonic acid (HEDPA),vinylidene-1,1-diphosphonic acid (VDPA) and1,2-dihydroxyethylidene-1,1-diphosphonic acid (DHEDPA).

It is noted that Zn(H₂) indicates the use of hydrogen formed in situ byuse of Zn in an acid medium. It is the nascent hydrogen that acts as thereductant.

No significant dissolution was observed for any of these reductants. Thefirst order rate constants were <1.0×10⁻⁶ sec⁻¹. Thus, the reductantsalone and the acids alone are not effective dissolution agents.

EXAMPLE 2

Goethite samples were treated at 25° C. with mixtures of 1M HEDPA andvarious reductant using the procedures described in Example 1. The firstorder rate constants (k) and induction times (ti) for dissolution of thesolid were measured. The results were as follows:

    ______________________________________                                        Reductant      k(sec-1)    ti(min)                                            ______________________________________                                        none           <1.0 × 10.sup.-6                                                                    not determined                                     0.1M hydroquinone                                                                            1.0 × 10.sup.-6                                                                     0                                                  0.1M NH.sub.2 OH'HCl                                                                         1.0 × 10.sup.-6                                                                     0                                                  0.1M SnCl.sub.2                                                                              2.3 × 10.sup.-5                                                                     0                                                  0.1M Na.sub.2 SO.sub.3                                                                       2.2 × 10.sup.-4                                                                     47                                                 0.2M Zn(H.sub.2)                                                                             5.0 × 10.sup.-4                                                                     23                                                 0.1M Na.sub.2 S.sub.2 O.sub.4                                                                1.9 × 10.sup.-3                                                                     0                                                  0.1M SFS       2.3 × 10.sup.-3                                                                     8                                                  ______________________________________                                    

Although all HEDPA/reductant mixtures provided rate accelerationsrelative to the individual components, the accelerations withHEDPA/sodium dithionite (Na₂ S₂ O₄) and HEDPA/sodium formaldehydesulfoxylate (SFS) mixtures were amazingly large (>2000 fold) as comparedwith the individual component.

EXAMPLE 3

Goethite samples were treated at 25 degrees Centigrade with 0.1Msolutions of SFS and various diphosphonic acids, using the proceduresdescribed in Example 1. The first order rate constants and inductiontimes for dissolution of the solid were measured. The results were asfollows:

    ______________________________________                                        Acid         k(sec-1)    ti(min)                                              ______________________________________                                        none         <1.0 × 10.sup.-6                                                                    not determined                                       HEDPA        2.3 × 10.sup.-3                                                                     8                                                    VDPA         2.5 × 10.sup.-4                                                                     0                                                    DHEDPA       3.4 × 10.sup.-4                                                                     0                                                    ______________________________________                                    

All diphosphonic acids gave large (2-3 orders of magnitude) rateaccelerations, with the HEDPA/SFS rate being the largest.

EXAMPLE 4

Goethite samples were treated at 25 degrees Centigrade with variousacids in the presence and absence of 0.1M SFS, using the proceduresdescribed in Example 1. The first order rate constants and inductiontimes for dissolution of the solid were measured. The results were asfollows:

    ______________________________________                                        Acid/Reductant   k(sec-1)  ti(min)                                            ______________________________________                                        2M HCl/none        2 × 10.sup.-6                                                                   0                                                  2M HCl/SFS       2.0 × 10.sup.-5                                                                   0                                                  1M Oxalic/none   5.0 × 10.sup.-5                                                                   49                                                 1M Oxalic/SFS    5.2 × 10.sup.-4                                                                   0                                                  ______________________________________                                    

Rate accelerations of only about 10 fold were observed with these acidsusing SFS as the reductant.

EXAMPLE 5

Goethite samples were treated at 80 degrees Centigrade with variousacids in the presence and absence of 0.1M SFS, using the proceduresdescribed in Example 1. The first order rate constants and inductiontimes for dissolution of the solid were measured. The results were asfollows:

    ______________________________________                                        Acid/Reductant    k(sec-1)  ti(min)                                           ______________________________________                                        1M HNO.sub.3 /none                                                                              4.8 × 10.sup.-6                                                                   0                                                 1M HNO.sub.3 /SFS 4.1 × 10.sup.-5                                                                   0                                                 3M H.sub.2 SO.sub.4 /none                                                                       5.7 × 10.sup.-4                                                                   29                                                3M H.sub.2 SO.sub.4 /SFS                                                                        1.9 × 10.sup.-3                                                                   0                                                 1M HEDPA/none     1.8 × 10.sup.-4                                                                   120                                               1M HEDPA/SFS      1.1 × 10.sup.-2                                                                   0                                                 ______________________________________                                    

When combined with mineral acids, rate accelerations of less than anorder of magnitude were observed: SFS gave rate accelerations of 8.5with 1M HNO₃ and 3.3 with 3M H₂ SO₄. The rate acceleration with 1M HEDPAunder these conditions was about two orders of magnitude at 78.

EXAMPLE 6

Goethite samples were treated under mild temperature conditions of 25degrees Centigrade with 1M HEDPA and 0.1M solutions of various reducingagents, using the procedures described in Example 1. The times fordissolution of the Goethite were measured and the times in minutes fordissolution of 10%, 50% and 90% of the solids were calculated. Theresults were as follows:

    ______________________________________                                        Reductant 10% (min)    50% (min) 90% (min)                                    ______________________________________                                        NH.sub.2 OH                                                                             >100         >100      >100                                         NH.sub.2 NH.sub.2                                                                       >100         >100      >100                                         Hydroquinone                                                                            >100         >100      >100                                         ascorbic acid                                                                           >100         >100      >100                                         Sn.sup.+2 >100         >100      >100                                         Na.sub.2 SO.sub.3                                                                         42         >100      >100                                         Zn (H.sub.2)                                                                              19           46      >100                                         SFS         5            13        27                                         Na.sub.2 S.sub.2 O.sub.4                                                                  1            6         27                                         ______________________________________                                    

The particularly high activity available with sulfur based reductantsand in situ formed hydrogen (nascent) can be seen from these results.

EXAMPLE 7

Goethite samples were treated at 80 degrees Centigrade with 0.1M HEDPAand 0.1 M solutions of various reducing agents, using the proceduresdescribed in Example 1. The times for dissolution of the Goethite weremeasured and the times in minutes for dissolution of 10%, 50% and 90% ofthe solids were calculated. The results were as follows:

    ______________________________________                                        Reductant 10% (min)    50% (min) 90% (min)                                    ______________________________________                                        NH.sub.2 OH                                                                             >35          >35       >35                                          NH.sub.2 NH.sub.2                                                                       >35          >35       >35                                          hydroquinone                                                                             14          >35       >35                                          Sn.sup.+2  3            12       32                                           ascorbic acid                                                                            1             3       9                                            Zn (H.sub.2)                                                                            <1             2       6                                            Na.sub.2 SO.sub.3                                                                       <1             2       5                                            SFS       <1            <1       3                                            Na.sub.2 S.sub.2 O.sub.4                                                                <1            <1       3                                            ______________________________________                                    

Although treatment under high temperature conditions as compared withExample 6, resulted in higher activities for all reactants, thesurprisingly more active reductants in accordance with the condition ofExample 6, maintained their comparatively higher activity under the morerigorous conditions of Example 7.

EXAMPLE 8

The ability to decontaminate metal substrates bearing radioactivecontaminants is shown by the following:

Sample Preparation

Stainless steel discs, approximately one inch in diameter, werecontaminated with ²³³ U, ²³⁹ Pu or ²⁴¹ Am (actinide series elements).

Each sample was prepared by 1) oxidizing the surface with hydrogenperoxide; 2) heating a disc to a dull red heat for a few seconds in air;3) depositing tracer level concentrations of one of the three actinides,as a nitrate salt, on the surface of the disc; and 4) heating the discagain to a dull heat for 10 seconds to insure that the actinide nitrateis converted to its oxide and to insure that the activity is driven intothe oxide film on the surface of the steel discs.

The sample discs were then counted radiometrically using an internalproportional alpha counter to measure the quantity of activity on eachdisc. These samples represent an extreme case with respect to difficultyin removing contamination.

Decontamination Testing

The discs were introduced into solutions of1-hydroxyethylidene-1,1-diphosphonic acid (HEDPA) and SFS as shown inthe following Table. The solutions were stirred with a magnetic stirrer.After the indicated predetermined period of time, the discs were rinsedwith water, dried on a hot plate and recounted in the proportional alphacounter. The discs were then leached a second, and in some cases a thirdtime, using the same procedure. Two series of experiments wereperformed, one at room temperature and the other at 70-80 degreesCentigrade. The results are tabulated below:

    ______________________________________                                        Leaching Solution: 1M HEDPA/0.1M SFS                                          Room Temperature (about 25 degrees Centigrade)                                Activity on Disc (Counts/Minute)                                              Contact Time                                                                              233.sub.U  237.sub.Pu 241.sub.Am                                  ______________________________________                                        none        1.7 × 10.sup.+3                                                                    1.0 × 10.sup.+5                                                                    1.0 × 10.sup.+5                       10 minutes  1.2 × 10.sup.+3                                                                    3.7 × 10.sup.+4                                                                    3.9 × 10.sup.+4                       20 minutes  1.1 × 10.sup.+3                                                                    2.6 × 10.sup.+4                                                                    3.8 × 10.sup.+4                       15 hours    6.9 × 10.sup.+2                                                                    2.6 × 10.sup.+4                                                                    8.3 × 10.sup.+2                       Total % removal                                                                           90         74         99                                          ______________________________________                                    

    ______________________________________                                        Leaching Solution: 1M HEDPA/0.1M SFS                                          70-80 degrees Centigrade                                                      Activity on Disc (Counts/Minute)                                              Contact Time                                                                              233.sub.U  237.sub.Pu 241.sub.Am                                  ______________________________________                                        none        7.3 × 10.sup.+3                                                                    1.2 × 10.sup.+5                                                                    1.0 × 10.sup.+5                       10 minutes  2.1 × 10.sup.+2                                                                    1.0 × 10.sup.+4                                                                    2.5 × 10.sup.+4                       20 minutes  8          9.4 × 10.sup.+3                                                                    1.8 × 10.sup.+4                       Total % removal                                                                           99.9       92         82                                          ______________________________________                                    

Discussion

Although there is an additive effect for dissolution of metal oxideswhen combining various acids with reductants, a synergistic effect isshown when di- and polyphosphonic acids are used. This includes theaminophosphonic acids such as aminotris(methylenephosphonic acid). Thereductants include organic reductants, such as ascorbic acid andhydroquinone and inorganic reductants. Sulfur-based reductant such assodium sulfite, sodium dithionite and sodium formaldehyde sulfoxylateshow the greatest effect. Hydrogen formed in situ shows a similar strongeffect.

Acid concentrations can range up to the concentration of standardconcentrated acid as manufactured, but are preferably in the 0.01-3Mrange. The reductant concentrations can range up to about 5M, but arepreferably in the 0.001-0.3M range. The ratio of acid to reductant ispreferably at least 0.5, more preferably at least 5 and most preferablyat least 10.

The treatment temperatures can range from ambient up to the refluxtemperature of the treatment solution., but preferably in the ambient to80 degrees Centigrade range. Treatment at ambient temperature isespecially preferred.

Methods of Use

To use the inventive composition for decontaminating hand tools or othersmall pieces of equipment which may have been contaminated such as byuse in a nuclear power plant, it is possible simply to dip the equipmentor tools into a container of a composition in accordance with thepresent invention. Alternatively, the composition can be sprayed ontothe surface or rubbed on with a suitable applicator.

Depending on the amount of contamination and the condition of theequipment, a contact time of several minutes should be used. A contacttime of 10 minutes up to about 1 hour is preferable. This can bedetermined by measuring the residual radiation of the tools to be surethat they are properly decontaminated. As shown in Example 8 above, evenwith baked-in contaminants, substantial reduction in contamination ispossible after 1 hour although some samples were treated for up to 24hours.

If it is desired to decontaminate pipes through which radioactive fluidshave passed, or just to remove iron oxides from the inside of the pipesthrough which water or steam has passed, essentially the same principleapples. A composition in accordance with the present invention iscirculated through the pipes or other equipment to be decontaminated fora sufficient time to accomplish the required results. Spraying orapplying with a suitable applicator may also be used. If equipment isbeing decontaminated, the level of radiation can be measured todetermine when the treatment is sufficient. The removal of scale orother iron oxide deposits can be determined by inspection or, in extremecases where there is clogging, by a reduction in the back pressure onthe materials being pumped through the system.

Removal of iron oxide contaminants can be done using usual equipment toleach contaminants from many types of materials. Heap leaching of soilor ore can be practiced, for example.

The treatment of radioactive sludge can also be accomplished by leachingthe sludge with the inventive materials although usual precautionsnecessary for handling such materials must be applied. Leaching can beaccomplished by heap-leaching or batch processing methods such as areusual in metal value recovery operations used in the mining industry.

The above is by way of illustration of preferred embodiments of theinvention but is not considered limitative of the scope of the inventionwhich is defined by the following claims.

What is claimed is:
 1. A method of dissolving a metal oxide comprisingcontacting the metal oxide with a composition comprising a di- orpolyphosphonic acid and sodium formaldehyde sulfoxylate, each beingpresent in sufficient amount to provide a synergistic effect withrespect to the dissolution of the metal oxide, the compositionoptionally containing corrosion inhibitors and pH adjusting agents. 2.The method of claim 1, wherein the metal oxide is an oxide of iron, anactinide or a lanthanide.
 3. The method of claim 1, wherein the metaloxide is an oxide of chromium, lead or manganese.
 4. The method of claim1, wherein the metal oxide is a surface layer on a solid substrate. 5.The method of claim 4, wherein the metal oxide is an iron oxide and thesubstrate solid substrate is iron or an iron-containing alloy.
 6. Themethod of claim 4, wherein the metal oxide layer contains radioactivematerials and the method is used for decontamination of the solidsubstrate by dissolving the metal oxide layer.
 7. The method of claim 1,wherein the metal oxide is present in a particulate substrate, and thecontacting comprises washing or leaching the particulate substrate toremove the metal oxide.
 8. The method of claim 7, wherein the metaloxide is an iron oxide, and the particulate substrate is kaolin orsilica.
 9. The method of claim 7, wherein the particulate substrate issoil which contains radioactive contaminants in the form of metal oxidesand the method comprises removing the radioactive contaminants bydissolving them in said composition.
 10. The method of claim 9, whereincontacting of the metal oxide with the composition comprises heapleaching or in-situ contacting methods.
 11. The method of claim 7,wherein the particulate substrate is an ion exchange resin.
 12. Themethod of claim 1, wherein the metal oxide is in radioactive nuclearwaste sludges and the method comprises partially or completelydissolving the sludge.
 13. The method of claim 1, wherein saidcomposition comprises 0.01M-3M of the di- or polyphosphonic acid, saidmixture having a molar ratio of the di- or polyphosphonic acid to thereductant of at least 0.5.
 14. The method of claim 1, wherein saidcomposition contains said corrosion inhibitors and/or a sufficientamount of said pH adjusting agent to raise the pH of the mixture to atleast
 7. 15. The method of claim 1, wherein the di- or polyphosphonicacid is VDPA, DHEDPA, methylenedisphosphonic acid ornitrolotris(methylenephosphonic acid).
 16. The method of claim 1,wherein the di- or polyphosphonic acid is HEDPA.
 17. The method of claim1 wherein the metal oxide is iron oxide, chromium oxide, lead oxide,maganese oxide, an oxide of an actinide and an oxide of a lanthanide.18. A method of recovering metal values from metal oxide containingmaterials comprisingdissolving the metal oxide by forming a complex witha mixture of a di- or polyphosphonic acid sodium formaldehydesulfoxylate, each being present in a sufficient amount to provide asynergistic effect with respect to the dissolution of the metal oxide;releasing the metal oxide from the complex and then recovering the metalvalues.
 19. The method of claim 18 wherein the metaloxide is iron oxide,chromium oxide, lead oxide, manganese oxide, an oxide of an actinide andan oxide of a lanthanide.
 20. The method of claim 18, wherein thecomplex is destroyed by treatment with an oxidizing agent to release themetal oxides.
 21. The method of claim 20, wherein the oxidizing agent ishydrogen peroxide.
 22. The method of claim 18, wherein the mixturefurther contains a pH adjusting agent or a corrosion inhibitor.